After six years of dating, and one year engaged, Carol, 41, of Winter Park, Fla., felt something was off with her fiance. She noticed his Facebook profile had a thorough recounting of his life but didn't mention her. She also noticed he had a new female Facebook friend from the Florida Keys, where he had recently gone on vacation while she stayed home — a house they owned together — to plan their wedding.

When Carol (whose surname is being withheld) expressed her concerns, he said he wasn't sure he wanted to get married, she said. So she gave him back the ring and told him to ask when he was ready.

That didn't make it any less painful when, later that month, she said, she snooped in his email and saw a long message from the aforementioned Facebook friend describing a sexual relationship that had turned emotional.

"My world was just rocked," Carol said. "I felt that I had lost my whole purpose."

No vows need be exchanged for cheating to devastate people who believe themselves to be in committed relationships, a steadily growing demographic as couples put marriage off longer or decide they don't need a wedding at all.

Forty-eight percent of women cohabited with a partner as a first union in 2006 to 2010, compared with 34 percent in 1995, and more unmarried couples are having children, according to a report this year from the Centers for Disease Control and Prevention.

Without formal legal ties, conventional wisdom would suggest it's easier to bail after infidelity tears through the trust and intimacy that hold a couple together.

And sometimes it is. But rarely is the post-affair path clear-cut.

The limited research that exists on committed but unmarried couples suggests infidelity in those relationships is slightly more common and accepted than in marriage. A study published this year by University of Denver researchers in the Journal of Sex Research followed a nationally representative sample of 993 unmarried individuals in committed opposite-sex relationships to explore predictors of first-time infidelity. It found that 14 percent had "sexual relations" outside of the relationship over a 20-month period, and 43 percent of those broke up after the infidelity.

People who cheated were most likely to have: parents who never married; problematic alcohol use; more prior sexual partners; lower relationship satisfaction; higher levels of negative communication or psychological or physical aggression in the relationship; lower dedication or no plans to marry the partner; and greater suspicion that the other partner was cheating. Notably, the frequency or quality of the couple's sex life did not raise the risk of infidelity, nor did cohabitation decrease it.

More research explores cheating within marriage, which has become less tolerated in society.

Eighty-one percent of people of all marital statuses polled in 2012 said extramarital sex is always wrong, up from an average of 70.5 percent of people who said so in 1973 to 1980, according to the National Opinion Research Center's General Social Survey, a widely used sociological poll based at the University of Chicago. A Gallup poll earlier this year found that marital infidelity was at the bottom of the list of morally acceptable behaviors — lower than human cloning, polygamy and suicide — with 91 percent of respondents considering it morally wrong.

Yet the prevalence of cheating among married couples hasn't budged much. About 19 percent of men and 12.3 percent of women in 2012 said they'd had sex with someone other than their spouse while married, about the same as it was 20 years ago (though there's been considerable variability year to year), according to the General Social Survey. That's likely an underestimate given the reluctance of people to admit to affairs and the survey's narrow definition of infidelity as sex.

Infidelity wreaks havoc whether you're married or not. But for unmarried committed couples, there often is a difference in reasons for and consequences of an affair.

"If there's less to lose, there's a greater chance that somebody will stray from that relationship," said Dr. Scott Haltzman, a psychiatrist and author of "The Secrets of Surviving Infidelity" (Johns Hopkins University Press), released this year. "And in part, layered in the decision to be a couple but not marry is the possibility and the openness to seek relationships elsewhere."

To be sure, there are many unmarried couples who are just as or more committed than their married counterparts and have just as much or more to lose, particularly if they have children or have mingled their finances.

But Haltzman said the lack of a ring often signals that people are still in an experimentation phase of the relationship and keeping their options open. When unmarried couples get in ruts, cheating can be a way of forcing the hand.

Just because someone begins to play the field doesn't spell the death of the relationship, Haltzman said. While partners should hold each other accountable when there are expectations of exclusivity, a lapse can open a conversation about where the couple wants to go and how to define the relationship, he said.

Blake Sharpe, 25, of Washington, D.C., said she has cheated in two different serious relationships, admittedly immature reactions to monotony or feeling that she was being mistreated.

Also, she said, "it's a fear of missing out. Even if you have something good, you still want to feel needed or wanted by other people, and that probably comes from some insecurity."